Update Change model to latest API ontology

According to the latest API ontology, property hasDescription in Change is optional xsd:string.

Change String description to Optional<String> in https://git.openlogisticsfoundation.org/wg-digitalaircargo/ne-one/-/blob/da53bb366434dfe572a04bbc247da0eb577ec4c9/src/main/java/org/openlogisticsfoundation/neone/model/Change.java#L16